No matter which direction an object moves horizontally on the earth, there will be deviations: the northern hemisphere is right and the southern hemisphere is left. This phenomenon is called the earth's rotation deviation. When an object is at rest, it is not affected by the geostrophic force, which is the result of the earth's rotation. When an object moves, it always tries to keep its original direction and speed because of its own inertia. The direction of geostrophic force is perpendicular to the direction of object movement, which has certain influence on the direction of object movement and makes it deflect to the right or left. The linear speed of the earth's rotation varies from place to place. In the northern hemisphere, when the air flow moves from north to south, that is, from the latitude with lower rotation speed to the latitude with higher rotation speed, the air flow will deviate from the meridian at the origin and shift to the right, that is, the original north wind will gradually become the northeast wind; The same is true of other situations. Objects moving horizontally on the equator will not be biased because the rotational biasing force on the equator is zero.

The wind is blown by the pressure gradient force. Unexpectedly, however, once the wind begins to walk, it does not move directly from the high pressure side to the low pressure side according to the direction indicated by the pressure gradient force, but keeps deflecting, deflecting to the right in the northern hemisphere and to the left in the southern hemisphere. This is an objective fact proved by numerous observations.

It can be seen that there must be some other force pulling it to turn from the side of the wind.

After people's in-depth practice and research, we finally found this force. This is geostrophic deflection. The name itself tells us that the force that causes the wind to deflect is originally caused by the rotation of the earth. On the constantly rotating earth, it is not only the wind that is affected by the geostrophic force, but all objects moving relative to the ground are affected by it, but because the geostrophic force is extremely small compared with other forces on objects, it is not noticed by people. However, after a long time, the geostrophic deviation left traces in some parts of the earth. It is found that along the flow direction, in the northern hemisphere, the right bank of the river is often steeper than the left bank; In the southern hemisphere, the left bank of the river is steeper than the right bank. This proves the existence of geostrophic deflection. There is little difference in the scouring effect of this water flow on the left and right banks, but the water in the river will flow day and night for 1000, 10000 and 100 million years.

Then how can the earth's rotation produce a biasing force?

To answer this question, let's do an experiment first:

Make a disk out of cardboard, fix the center of the disk so that it can rotate, and then prepare a pencil and a ruler. Put the ruler on the disc and take it in any direction. Then let the pencil advance on the disk near the edge of the ruler. At this time, the nib left a mark on the disk. AB is of course a straight line. This shows that on a non-rotating disk, the moving nib completely follows the direction of your hand's force, without bias interference.

However, if the disc is rotated and the scale remains in its original position, the deflection force will immediately show its effect. Ask the assistant to turn the disc counterclockwise, and you will still move the pencil tip forward next to the edge of the ruler as before. You can try it in all directions, up, down, left and right. When the pen tip runs from the starting point A to the point B on the edge of the ruler, the disc has rotated by an angle, and the starting point A of the pen tip above and below the disc becomes A', so that the mark A'B left by the pen tip on the disc is not a straight line, but a curve that constantly deflects to the right. If your assistant turns the disk clockwise, the footprint left by the pen tip on the disk is a curve that constantly deflects to the left.

At this time, for the ruler, the movement of the nib is always a straight line, because it never leaves the edge of the ruler! But for a rotating disk, the motion of the pen tip is obviously curvilinear.

The earth is spinning all the time, and the earth on which people step is like a big rotating disk. Looking down from the North Pole, this big disk runs counterclockwise; Looking down from the South Pole, the direction of this big disk is clockwise. The deviation of air-wind walking on this big disk is due to the relative motion between the wind and the rotating ground. Long-term flowing water can show the effect of cross-strait drift, precisely because of their relative movement with the rotating ground.

In this way, the wind deviates from the direction of the pressure gradient force, and no real force is at work. The geostrophic deflection force is just a fictional force for the convenience of studying this deflection phenomenon. This imaginary force is perpendicular to the wind direction, pointing to the right of the wind direction in the northern hemisphere and to the left of the wind direction in the southern hemisphere. Because it only shows that there is relative motion between the air and the rotating ground, rather than the actual force acting on the air, it can only deflect the wind direction, but can't start the wind, and can't change the already started wind speed. Wind speed depends on air pressure. If the pressure gradient force is equal to zero, no wind can be generated, so there is no relative motion with the ground, and the geostrophic deflection force does not exist. With the pressure gradient force, wind will inevitably be generated correspondingly, and the wind will also generate geostrophic deflection force. The greater the wind, the greater the geostrophic deflection force.

Under the pressure gradient force, the wind is pushed to the low pressure side. Once the wind begins to move forward, it immediately generates a geostrophic deflection force, which pulls the wind to the right (as shown in the left picture). The speed of the wind is accelerated under the continuous push of the pressure gradient force, and the stronger it blows, the greater the geostrophic deflection force, which pulls the wind to deflect to the right (such as the lower right). Because the direction of the geostrophic bias force is always perpendicular to the wind direction, while pulling the wind direction, the geostrophic bias force itself is constantly deflected to the right, that is, it turns more and more in the opposite direction to the pressure gradient force. When the wind direction makes an angle of 90 degrees with the direction of the pressure gradient force, although the pressure gradient force still exists and has the same magnitude as before, the effective component of the wind direction is equal to zero, so the wind is no longer driven by inertia to move at the same speed. At this time, the geostrophic deflection force just turns behind the pressure gradient force, and the contradictory sides are equal in size and opposite in direction. From the previous unbalanced state to the balanced state, the wind direction is no longer deflected. It is obvious from the figure that in the state of equilibrium, the wind direction is parallel to the isobar.

Since the discovery of this law of equilibrium, it has brought a lot of convenience to meteorologists. The relationship between air pressure and wind has become so close: knowing the distribution of air pressure, we can infer the distribution of wind; Similarly, knowing the distribution of wind can also infer the distribution of air pressure in turn. In order to facilitate memory, the relationship between air pressure and wind is summarized as follows: wind speed is directly proportional to air pressure gradient; The wind direction is parallel to the isobar. In the northern hemisphere, it stands in the leeward, with high pressure on the right and low pressure on the left. The southern hemisphere is the opposite.

For example, in the distribution of air pressure, the isobar near Beijing runs from southwest to northeast, with high pressure in the southeast and low pressure in the northwest. According to the laws summarized above, it can be inferred that the southwest wind blows in Beijing, while the isobar near Shanghai runs east-west, with the north high and the south low. According to the law, it should be blowing east wind. Look again, the isobar near Shanghai is thinner than that near Beijing, so the wind in Shanghai should be smaller than that in Beijing. Another example is the north wind blowing in Beijing and the south wind blowing in Shanghai. According to the law, the isobar distribution near the two places should be north-south, but the pressure on the west side near Beijing is higher than that on the east side, and it is the opposite in Shanghai. Moreover, because the wind force in Shanghai is greater than that in Beijing, the pressure gradient near Shanghai is greater than that in Beijing, and the isobar is denser than that near Beijing.

The atmosphere is like an automatic regulator, and the balance and imbalance between the pressure gradient force and the geostrophic deflection force can be automatically adjusted. Although it is difficult to achieve absolute balance, the actual wind is also difficult to keep absolutely parallel to the isobar, but the wind direction is always on the isobar, not too far. Therefore, the theoretical wind is very similar to the actual wind, and the relationship between air pressure and wind has always been regarded as the law of atmospheric movement by the meteorological stations.